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Icing and airspeed
I have searched the subject of "when to deploy deicing boots" and have found no comments on the airspeed required during deicing operations. If you have 1/4 to 1/2 inch of ice on the leading edges of your aircraft, how much ice do you have on the surfaces behind the trailing edge of the ice boots?
Getting rid of the ice on the boots only gets that ice off the surface of the boot—it doesn't get the ice off the rest of the wings, etc. The rest of the ice is blown off the surfaces behind the boots, but only if the aircraft is moving fast enough and the surface is smooth enough to let the ice slip off. That 1/4 to 1/2 inch of ice sticking up on the front edge of the airfoil has to be very bad for the smooth flow of air required for lift, especially at slow speeds.
If my memory is correct, the aircraft carrying Sen Paul Wellstone (D-MN) came down through clouds from a high altitude. It is my feeling the pilot was slowing down so he could land, encountered a large ice buildup and deployed the boots. He was probably not going fast enough to blow the ice off the rest of the surfaces, went into an immediate stall and went almost straight down.
A very similar situation occurred in a twin Beech about 18 years ago out of BFI. The pilot was climbing out in icing conditions, allegedly deployed the boots and, because the ice that stayed on the wings created an immediate disruption to the airflow, caused an immediate stall. That plane crashed into the side of a low mountain. The investigation developed the information that there was ice on the wings behind the boots and that the site was below freezing.
Similar condition crashes occurred with Sen Mel Carnahan (D-MO) on board and a feeder line crash at PSC (Pasco WA) about 20 years ago. Is the common thread going too slowly?
I have not seen any mention of airspeed and its effect on ice when deploying deicing equipment from NTSB or other information. I do not make this a daily endeavor but think about it each fall. Nor have I seen any mention or suggestion that the aircraft operator either paint the top wing surfaces with some of the new paints that almost prevent anything from sticking to the paint or put a good coating of a paint preservative on the plane to reduce the ability of ice to hang on to the surfaces.
Think about how easy it is to get the ice cubes out of smooth plastic trays as opposed to the old aluminum trays, which were smooth to the touch but grabby to the water/ice. Remember that most aircraft have been out in the sun for the past 8 months and the exposed surfaces have been cooked dry. Your clean, freshly waxed car is much easier to wash than your dirty, unwaxed car, isn't it?
_ATP. Type ratings not specified
- Current aircraft icing information and training does include information about airspeed—in fact, many aircraft have a minimum speed in icing conditions to protect portions of the aircraft susceptible to ice accumulation. If the ice remains on the wings behind deicing boots, the airflow is disrupted and can raise the stall speed of the airplane. So yes, airspeed is important and the pilot must keep the speed up in icing conditions to avoid a stall situation for multiple reasons.
However, going faster is not the only answer. You can only go so fast—Vne/Vmo. Avoiding icing conditions in the first place is the best practice. When it can't be avoided, the pilot should exit icing conditions as soon as possible. Just because an aircraft is equipped for known icing conditions does not mean continued flight in known icing is a good idea. Deicing equipment on an aircraft should get you out of icing conditions, not get you in it.
That being said, on Aug 10 this year FAA issued a new proposed rulemaking for new aircraft icing certification requirements. The changes are based mostly on NTSB recommendations from accident investigations involving icing conditions over the years and will apply only to newly certified aircraft. According to FAA, more than 100 separate safety mandates concerning icing hazards have been issued that current rules don't adequately address.
According to FAA and NASA, a pilot should use deicing boots as soon as ice becomes apparent and not wait until 1/2 inch of ice has accumulated. Research continues on the underlying safety concern of flight in freezing drizzle, super large droplets (SLDs), ice crystals and the potential handling and performance issues they may cause. Until then, pilots must remain vigilant to the conditions being flown in and avoid prolonged operations in an icing environment.